In the conventional process of surface mount technology rework, prototyping, and research and development, exchange or replacement of single components is often encountered. Presently some of the common methods being used to apply solder for placing replacement components onto populated printed circuit boards include:
1. Use of existing solder to mount new component PA1 2. Deposit a solder paste bead onto substrate pads PA1 3. Use of solder tape over substrate pads PA1 4. Surface tension transfer of solder to substrate pads PA1 5. Dispensing of solder dots onto substrate pads
Except for solder dot dispensing which requires costly equipment or if done manually is very tedious, the existing methods often exhibit placement inaccuracy, contamination and inconsistent paste volume from pad to pad resulting in problems in bridging and poor solder joints. It is therefore the object of this invention to provide a functionally simple, consistent, repeatable and economical method and apparatus for solder paste application in single component placement processes.
The object and advantages of the present invention will become apparent from the following description and accompanying drawings. The invention relates basically to a simple apparatus that enables available etched stencil plate and solder paste stenciling technology to be used in small and limited work production situations where commercial paste screening or stenciling is not viable.
The apparatus utilizes a stencil mounting head attached to a pivot arm assembly providing easy visual alignment of the stencil plate and substrate pads. This alignment is maintained by downward biased spring force of the stencil head against the substrate surface allowing paste dispensing and squeegee procedures to be completed without disturbing the stencil alignment. The printing procedure is completed by the lifting of the stencil plate from the substrate surface leaving the solder paste precisely applied. Control and isolation of external movement during the critical lifting operation is accomplished by an internal central lifting rod operated by a flexible cable mechanism. Various stencil patterns can be easily interchanged through the use of the associated stencil plate holder that slips onto the mounting head.